PALEOCLIMATE AND PALEOECOLOGY OF CENTRAL UTAH DURING THE PAST 200,000 YEARS FROM A SOIL CARBONATE PENDANT

Understanding the magnitude and duration of past climate events is essential to make accurate predictions about how global climate will vary over the next century. Quantitative paleoclimate records from arid regions are difficult to obtain, often present complex growth histories, and have poor chronologic dating relative to marine sediment archives. Pendants, which are several centimeter thick coatings of soil carbonate formed on the bottoms of boulders, represent a novel source for soil and climate records in arid areas. We developed two high resolution stable isotope paleorecords (C and O) from pendants collected in Utah at Torrey and the University of Utah Rio Mesa research station ≈200 km east of Torrey. The pendants are dense, finely laminated, and formed at >20 cm depth. They were dated by the cosmogenic ³He, U/Th, and radiocarbon methodologies.

The pendants have nearly identical C isotope records with a roughly sinusoidal shape. The most negative values (-4‰ VPDB) occur during the Last Glacial Maximum (LGM). Plants using the C4 photosynthetic pathway dominate through the last 200 ka, as demonstrated by δ¹³C values of -4 to -1‰ (≈70-100% C4 plants). The consistency between these two records meets theoretical expectations that the sites, which sit at similar latitudes, should show similar changes in paleoecology and paleoclimate through time.

The oxygen isotope record at Rio Mesa is dynamic and does not match the Torrey record. It ranges from -7 to -13‰ (VPDB) and shows a consistent increase towards the present. Just after the LGM there is a 1.5‰ spike in the record. These patterns are in strong contrast to the Torrey record, which stays between -8 and -9‰ for the entirety of the record. The Torrey record can be reasonably explained through isotopic effects caused by factors like ocean temperature, glaciation extent, and temperature of calcite formation. However, these factors are inadequate to explain the long-term 6‰ and short-term 1.5‰ shifts observed in the Rio Mesa record. The C isotope records indicate both sites experienced similar paleoclimate, which argues against, for example, differences in rainfall amount or intensity. Instead, these data point to a regional, long-term change in water source or seasonality of calcite formation as Rio Mesa currently exhibits a larger seasonal O-isotope range than Torrey.